In the related art, for example, a gas concentration calculating device for calculating a concentration of a gas such as carbon dioxide has been introduced in fields of an air-conditioning system, and so on. As ON/OFF of ventilation is controlled based on calculation results in the gas concentration calculating device, the air-conditioning system is efficiently operated and power consumption is reduced. Such a gas concentration calculating device uses an NDIR (non-dispersive infrared) method, and the NDIR method is a technique of calculating a concentration of a gas based on attenuation upon passage of infrared light through a target gas.
As the gas concentration calculating device using the NDIR method, for example, Patent Document 1 discloses a gas concentration calculating device, in which light from a single light source is emitted into a gas cell, and the light passing through the gas cell is detected by a first detector and a second detector. The first detector detects light passing through an optical path constituted by a region for gas to be measured, and an inert gas region hermetically enclosed in a measuring gas chamber. The second detector detects light passing through an optical path constituted by a region for gas to be measured, and a gas region having the same gas as a gas to be measured, which is hermetically enclosed in a comparison gas chamber. In addition, an increase or decrease in irradiation light quantity is detected by the second detector, and an output of the first detector is calibrated.
Further, Patent Document 2 discloses a gas concentration calculating device for detecting a concentration of a sample gas in a cylinder. Here, a reflecting mirror is installed at a head of a piston reciprocating in the cylinder, and a light source and a detector are disposed at the head of the cylinder to be directed inward with respect to the cylinder. According to the above-mentioned configuration, light emitted from the light source and reflected by the reflecting mirror on the piston is received by the detector. According to the reciprocation of the piston, since an optical path length from the light source to the detector via the reflecting mirror is varied, an energy value received by the detector is varied. Thus, based on a variation in output values output from the detector, a concentration of the sample gas is calculated.